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The National Federation for the Blind says its members are unable to sign an e-petition calling for printed material to be more accessible to the visually impaired because of "Captcha" security.

A Captcha is a graphic or sound of a random word or number users must key in to prove they are human.

The petition, on the White House website, has received just 8,200 signatures.

The White House says the site complies with US accessibility standards.

Chris Danielsen of the US-based Federation for the Blind told the Politico website that he realised there was a problem after publicising the petition.

"We had asked people to sign the petition and we're getting these emails saying that people can't," he said.

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"The constitution allows all of us to petition our government for a redress of grievance. It says nothing about needing to be able to see in order to do so."

Damon Rose, editor of the BBC's Ouch blog for people with disabilities, said: "Captcha graphics are a nightmare - visually impaired people use screen readers to interpret their computer rather than their eyes and the screens can't manage them.

"Ironically if I see an audio capture I tend not to bother with it because it's usually such a poor experience... some of them sound like aliens talking and they put weird background noises over them. They are a bit of a joke in the blind community. I've spent half an hour on some and had to give up."

As a result, many visually impaired people could not contribute to debate and discussions on messageboards and blogs, Mr Rose added.

At the beginning of the year, ticketing service Ticketmaster removed Captchas from its sales website.

"It is generally speaking the one of the most hated pieces of user interaction on the web," said Aaron Young, from user experience consultancy Bunnyfoot at the time.

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